I’m looking to buy a used droboS (i would upgrade to 5d or 5n but my imac doesnt have TB) currently Have an fs (Im not a satisfied owner, so dont want to pay retail price, but willing to give it another try) and looking for some attached storage. Since I didn’t see a for sale forum i figure i’d post here anyone upgraded and looking to sell old equipment??
your imac doenst have TB … so you cant buy a networked drobo? i dont quite follow that logic…
and the 5D also has USB 3 - even if you only have USB 2 , i would still buy this one since it has a much faster processor than the 5D which helps with rebuilds.
just some thoughts (sorry i dont have a S for sale)
the benefit of the 5d are not there without TB. the 5N is promising but my imac doesnt NIC support jumbo frames so lack of performance. I have a fs and have lost everything 2x luckily i have backup in the safe to restore.
I’m running a Drobo 5N without Jumbo Frames (my 2010 MacBook Pro also doesn’t support them…), and as long as nothing else is a bottleneck (SSD source, Cat6 Gigabit network) I get over 100MBps (800Mbps) read/write speeds to the 5N. I can’t say it will address all of the issues you may have had with the FS, but rest assured performance is no longer the issue it used to be.
thats what i’ve read 90+ MBps, I am interested im at about 20 -30 with the FS but do I has some reservations about DR and past experiences I would love to give them another ry but not at 600 dollars plus drives.
You could just migrate your disk pack from the FS into a 5N as others have already done. No need to back up, configure a new array and restore to it. That may or may not be a convenience worth some money to you. 
Are you looking for Drobo S v1 or v2? Both esata, but v2 with USB3.0
The fastest connection on my iMac is fw800 so probably any droboS would be fine
I’ll sell you my Drobo S WITH 5 2TB drives for $650. I’ll do it for $500 without the drives.