I have, on several occasions, powered down, removed the cover from my beige G3 DT, disconnected the IDE CD_ROM drive, connected an IDE drive using the CD's drive and power cables, left the IDE drive just sitting atop the CD drive, and reconnected the power (leaving the cover off).
This configuration allows me to temporarily have an extra IDE HD and no CD-ROM.
This is very useful when upgrading drives and transferring data from an old drive to a new one - I just run the Mac like this for as long as it took to copy the old drive's data to the new one and assure mice-elf that the transfer was complete and none of the data was damaged in the transfer.
Then, when I was ready, I removed the old drive (in this model it's mounted on the bottom of the chassis) and installed the new one in its place.
As long as you keep the old drive intact and around for awhile, you can always repeat this procedure, connecting the old drive where the CD-ROM is, if it turns out that anything was lost or damaged and needs to be re-copied.
The beige G3 DTs also have an internal SCSI bus, and it seems to me that using old 250, 320, and 500 MB SCSI drives (connecting them temporarily, similar to the procedure I described above for the IDE drive, just long enough to copy the data, then storing them somewhere outside the Mac) for 'safety copies' of critical data is a Good Idea.
- kbeartx