These are all the suggestions I can come up with at the moment :
1. According to Microsoft, VIA USB Chipsets VT83C572 & VT82C568 are not supported under windows 2000, nor are VIA USB revisions earlier the rev 5. I don't know which via chipset you have, but if it turns out to be one of these, you may be able to get a patch from VIA's website.
2. You siad the drive caddy works in other machines, have you also tested the USB cable on other USB devices or other systems?
3. There is an issue in windows 2000 with the embedded usb driver & ntfs file system driver. The fact that you have SP4 loaded means that you should have the updated drivers running but it's possible they have been overwritten by older versions at some point. Check the version numbers of these files :
usbhub.sys v5.0.2195.3568 or later
ntfs.sys v5.0.2195.6920 or later
4. The file named as being the cause of the 0x50 error, is pqv2i.sys. This file is part of Symantec/Norton software, usually ghost I believe. But if you're sure that you don't have this loaded, then it can't related to that.
5. The fact that you experience the STOP error whenever you connect the drive, suggests that this is an enumeration fault. It may be that there is a patch for it but you would need to know the manufacturers name for the USB drive.
6. The only other solution that may or may not work, would be to install a PCI USB card & connect the drive to that.