Removable Disk Drives

9/9/98 Update

It appears that Windows 95, 98 and NT4 provide some support for large removable disk drives. NT will still not boot and run from a removable drive. There are also problems with some drives as described below.

11/14/97 Update

It appears that this problem is related to certain brands of removable disks. If you check the Fujitsu web site for technical Q&A for their 640 MB MO drive you will find a switch setting that goes to either Mac or PC. This switch appears to correct the problem listed below. This is a new drive and older drives may not be correctable.


I take back some of the good things I had to say about Microsoft Windows NT on previous pages. I tried NT version 3.1 when it first came out and found a major problem - lack of support for removable drives. As part of my continuing evaluation, I purchased a copy of NT 4.0 and proceeded to install it on one of my removable drives. I didn't want to mess up my hard drive and even shut it off so the installer wouldn't muck with it. Well, Win NT would run but it wouldn't install any programs. I called up Microsoft to find out what was going on and used up my only free support call finding out NT won't run properly on a removable drive - even if you set the Adaptec AHA-2940 BIOS to treat the drive as a fixed drive!

Anyone using a computer for desktop publishing needs removable drives. In fact, I prefer them for backups. I hate the idea of putting data on a cassette tape and prefer magneto-optical media because it can be used directly as a disk and is the best backup medium.

As it turns out, Win NT does support removable drives to a point. They do support FAT volumes but not NTFS. Perhaps they will say they support them because you can use one. You just need to shut the system down every time you want to change a disk!

All modern operating systems have MOUNT and UNMOUNT commands that allow you to disconnect a disk from the file system. Why doesn't Win NT have them? If Microsoft is going to push Win NT as THE CORPORATE OS then they better get with it. The Mac OS has this feature, and I believe most Unix systems support it. This is a glaring fault of Win NT and Microsoft should fix it. I doubt that they will because I believe it would require a major redesign of some of the NT sub-systems. They never included mount and unmount support in the Win32 API from the start and I believe adding it would require some major work. I caution any company planning on using Win NT to consider this problem before using NT.

I have also found problems with Windows 95 and NT when using removable drives. I have a Fujitsu M2512A 3.5 inch MO drive connected to an Adaptec AHA-2940 card. If I remove a disk, put another one in and click the disk icon in the Explorer tree pane it still shows the files on the old disk. If you go ahead and try to use the disk you will corrupt it! If I use the Eject command to eject the disk, the problem is still there, so the Eject doesn't flush the disk buffers. Apple was criticized for using the trash can to unmount volumes, but here we have a problem with Win 95 that could cause novice users to lose data!

The only solution I have found is to click the drive when it is empty to force it to see that no disk is installed (it then supplies the retry/cancel error box) and then put the new disk in. Another brand of MO drive doesn't have this problem, so it appears to be hardware related and is possibly a problem with the way data is cached. I haven't fully tested this with Win NT yet but it uses the same SCSI mini-port drivers and may also have this problem. What testing I did with NT showed similar problems and my solution for 95 didn't seem to work properly.

Here are the Microsoft documents that reference these problems:

From Windows NT 3.51 Resource Kit 

Chapter 5, Comparing NTFS with HPFS and FAT

For example, suppose Joe has a removable hard disk on his computer. That hard disk is formatted as an NTFS volume and has security permissions that allow access only to Joe and to one other coworker in his domain, Ann. Ann works at the company's branch office. Joe removes the disk from his computer and sends it to Ann, who installs it in her computer. When she accesses the files on the disk, since Ann's computer is in the same domain as Joe's, she sees that the security mechanisms within the domain for the NTFS volume are intact. 

Note Be sure to shut down the system before removing a disk containing an NTFS volume.

You can also look this up in the Microsoft Knowledge Base

Q101626 Windows NT Removable Media Requirements and Limitations

© James S. Gibbons 1987-2015