Since you already have some mobile hard drive trays - an even greener solution might be to stick the existing trays and drives in an old existing machine and use that as a backup server over network.
That might be an option I should consider.
Currently I'm the sole user of these boxes. With new hardware and virtualization, and one of the IDE-to-SATA converter mobile trays, I would not need to reboot my new box. I would need to reboot my NT4 Workstation because that guest OS never would see the new device and never supported hot swapping either. But "rebooting" a virtual guest OS is not a hard reboot but a soft restart of the guest OS. As the kids today say: easy-peasy.
Unlike Windows with its infamous file locks, backing up a GNU/Linux box in real time is not an issue. So my thinking right now is that I buy an IDE-to-SATA tray, pop in one of my IDE drives, hotplugging services see the new disk, and I run my backup script. I also concurrently restart NT4 and perform my backups there too.
I realize I could have GNU/Linux backup NT4 through the virtualization network layer, but those Windows file locks still prevent real time backups without special software that is granted those rights. My long standing solution was much easier. Because I had to reboot to insert the second IDE drive, I merely booted to my alternate NT4 C: partition and then backed up my primary C: drive from there. No file locks.
Another problem with backing up through GNU/Linux is restoring from backups. In such a case, those Windows files would be on ext3 partitions, not NTFS. So even if my virtual NT4 OS crashed hard and I needed to restore from backup, I'd want to do that from native NTFS partitions. Yes, there is today better support for NTFS through FUSE and NTFS-3g, but as far as I know, they do not support the older NTFS-4 format.
Initially I do not want to recreate the wheel. My Windows backup scripts are all well tested through the years so why not just keep using them? Ditto for my shell scripts for my Slackware backups. I already have my backup disks formatted so maintaining my current process seems the least resistive route.
With that all said, performing the backups through a network layer from one of my older boxes does maintain some appeal, however, if only for the geek or "cool" element. I do understand your reasoning about not buying the IDE-to-SATA tray and just use what I already have with no extra expense. That would be "green." I would have to put some thought into how I would need to revise my backup scripts, however, because they never were designed with networking in mind. I will spend some time thinking about that option before buying my new hardware.
I have mixed thoughts about retiring my old boxes because they are perfectly usable. My old 486 with 16 MB of RAM also is perfectly usable and yet has sat on the shelf for several years. At one time I considered converting the 486 into a firewall, but the box is completely ISA. To my knowledge ISA never supported 100Mbps, only 10Mbps. That speed is sufficient for the ISP, but way too slow for my LAN, which is at 100 Mbps. I'd have to buy a 100 Mbps switch to serve the network.
I could use one of the current two boxes for a firewall, which already support 100 Mbps network cards, but my Linksys WRT54GL with DD-WRT already does that and at a trifle of the energy consumption of a PC. So unless I discover a way to revise all of my backup scripts to a network box, I suppose the current two boxes will see little action other than occasional experiments. And that
is not at all a green approach.
Still, just chatting about these ideas gets the brain moving in the (eventual) correct direction!