When I looked around regarding your problem I found the following article which seemed to indicate you still needed the synth termination when using an internal drive. You mentioned you had removed it. That may well be the problem.

(Here is some info on K2000 SCSI from http://web.archive.org/web/20000915092041/http://www.xavax.com/bklein/k2000.htm )

More on Hard Drives, ID's, and SCSI
As I stated above, the power supply is really maxed out if it has to power a hard drive in addition to the synth and it's options/memory. It gets damn hot as it is. Many hard drives will require close to a half an amp on the 5 volts. Motor starting currents on the 12 volt line can approach 2 amps or more. You also have to worry about powering the required fan. You might want to think about providing a separate external supply for the drive. Laptop battery eliminator supplies that you may be able to find surplus should do fine.

Each device on the SCSI bus has to have its own unique ID selected, usually by a jumper or switch. It doesn't matter what the ID is - as long as each is different. You can select one of 7 ID's. Your synth will tell you what is connected and each device's ID. Terminators have to be physically on the ends of the SCSI cable. If you have an internal hard drive and don't use the SCSI port in the back, the synth's terminator needs to be installed (normally is) with another on the hard drive. If you do plan to use the SCSI port you probably need to remove the terminator on the synth board, terminate the drive, and have some kind of terminator that can plug the DB25 connector when a cable is not used (with a terminator at the device/controller at the end of the cable that plugs in here). Yes, it gets messy!

In addition to termination and ID hassles, you may have additional problems with capacity, compatibility, and types of SCSI busses. SCSI drives at 750mb and below are generally compatible capacitywise. Larger than that depends on your synth's level of firmware. Contact YC to get the details - you may be able to use a larger drive but just not use ALL of it. Several drives - hard drives and CDROMs - just plain don't work due to the SCSI chipset they use. I had trouble with a CDROM with an NEC controller chip. Best post a message on the K2000 mail list to check compatibility. Finally, take a look at the connector on the back of your unit. It is called a DB25. Newer SCSI drives and controllers use a different type connector style, called SCSI II (two). This is a smaller, thinner connector style. There are adapters to interchange these and the older cables/connectors but they aren't cheap. If you are recording you may want to disconnect the SCSI cable from the synth and pc to make things quieter.