1. You must try to find any 3rd party sellers that may still have stock or refurbished of the same exact make and model.
2. Make and model are very important but if the model is no longer available, you must stick with the same manufacturer and attempt to match hard drive specifications. If the drive is Toshiba, replace with a Toshiba drive. If the drive is Western Digital, stick with Western Digital. Do not mix manufacturers.
3. The speed of the replacement drive is very important and cannot be any different from the other drives. Example: if the current drives are 7200RPM, stay with 7200RPM. Do not purchase drives of different speeds.
4. Drive space can be larger. Ex: If you have 2Tb drives, you may go with a larger drive like 4Tb or 6Tb.
5. Drive Cache will not make a difference. 64MB vs 128 will not matter.
6. Drive Block size will matter. You must search on the manufacturer websites to find what the block size is on the current drive and compare to the possible replacement drive. If the current drives are 4K block sizes, you may use 4K or 512E block size drives. If the current drives are 512E, you can only use 512E block size drives.