There is no denying that dynamic disks have a lot of advantages over basic disks. Dynamic disks allow you to improve performance by striping a volume across multiple disks. They also give you the option of extending a volume to include unused space on other dynamic disks within the system.
In spite of all of these benefits however, there are situations when you should reconsider converting a basic disk into a dynamic disk for risk of problems. Here's a closer look at some scenarios when it's not in your best interest to convert basic disks to dynamic disks.
When you want to downgrade
This particular situation may not apply to most people, but it should be addressed. Dynamic disks are compatible with Windows 2000, Windows XP and Vista. However, if you are running XP and you were planning to downgrade to an even older operating system, such as Windows 98, then you shouldn't convert your basic disks to dynamic disks. This conversion process disables your ability to uninstall Windows XP and revert to your previous operating system.
If you're running a multi-boot configuration on a computer, you should not convert basic disks to dynamic disks. Windows XP, Windows Vista and Windows 2000 all support dynamic disks; however, you will cause problems if you upgrade a basic disk to a dynamic disk if you have all of these operating systems installed on the same machine.
Problems will occur when you perform the upgrade because Windows also updates certain registry entries to make the operating system aware of the presence of a dynamic disk. If these registry entries are not updated, then Windows will not know how to deal with the dynamic disk. If you convert a basic disk to a dynamic disk in a multi-boot environment, then the conversion will work fine for the operating system that you are using at the time of the conversion. However, the other operating systems that are installed on the computer will not be aware of the conversion, and will no longer recognize the disk.
Using legacy operating systems
Previously, I mentioned that older operating systems, such as Windows 98, Windows 95, Windows ME and Windows NT, are not compatible with dynamic disks. These OSes are so old, that their incompatibility with dynamic disks probably isn't going to be an issue for the majority of people out there. What you should consider though is that there are some versions of XP that do not support dynamic disks either.
This limitation ties in to the restriction on multi-boot environments. This situation, I believe, is a bit more significant. In a multi-boot environment, if an OS at least supports dynamic disks then an operating system may not recognize a disk once it has been converted to dynamic; Windows does, however, provide mechanisms that you can use to import the now "foreign" disk into the operating system.
In the case of legacy OSes though, the older OS will not be able to recognize the dynamic disk at all. If it is the boot volume that has been converted, then you will not even be able to boot the older OS after the conversion has taken place.
Another disk conversion situation that is not recommended is when the disk contains alternate partitions that were not created by Windows. For example, many computers ship with OEM partitions on the system drive. If you were to convert these disks to dynamic disks, then the OEM partitions become inaccessible.
Keep in mind that this limitation doesn't just apply to OEM partitions. The same limitation applies to any partition created by a non-Windows operating system. For example, I know plenty of techies who multi-boot Windows and Linux off the same computer. Linux creates non-Windows partitions, so if you were to convert a drive containing a Linux partition to a dynamic disk, Linux would no longer function.
The majority of the situations that prevent you from converting basic disk to dynamic disks are related to compatibility issues with alternate operating systems. Even within a single operating system there are some conditions that prevent you from making the conversion. For example, if you intend to install the FAT file system on a drive, you will not be able to convert that drive to dynamic as only basic volume supports FAT.
The conversion to a dynamic disk is also typically disabled for removable drives. Refer to my earlier remark about the conversion process updating registry entries. If you were to convert a removable drive to a dynamic format, then only the computer that was used to perform the conversion would recognize the changes. As such, Microsoft prevents you from converting removable drives to dynamic.
As strange as it may seem, there are situations in which you can not convert a drive on a laptop to dynamic. This limitation does not exist in Vista, but XP does prevent you from converting laptop drives to dynamic. Keep in mind though that this limitation is tied into the Advanced Configuration and Power Interface (ACPI) architecture. Therefore, if you have a really old laptop that is not ACPI compliant you can probably get away with converting a basic disk to a dynamic disk through Windows XP.
Finally, you will not be able to perform the conversion if the disk is full. Windows requires space at the end of the volume to maintain volume information. If the space is not available then the conversion process will fail.