When you say you "slaved it", let me check how. On any IDE port and cable, you can have up to 2 devices. The one on the end of the cable MUST be jumpered to be the port's Master. If there is a second device, it must be jumpered to Slave, and plugged into the middle connector. If that's what you did, good. BUT if you connected the old HDD to a DIFFERENT IDE port so that it is the ONLY device on that second port and cable, then it MUST be set to Master of that port.
Your post is confusing, because it says the drive is and is not found in Device Manager. I suspect you mean Disk Management in one case.
In case you have not used Disk Management before, start this way. Click on Start at bottom left and then RIGHT-click on My Computer and choose Manage. In its tree at left choose Disk Management.
In Disk Management there are two panes on the right, and each SCROLLS so you can see all it has. Concentrate on the LOWER RIGHT pane which shows all hardware drives in the system, even those Windows cannot use yet. Since your old IDE is shown in BIOS, it should be here in Disk Management's Lower Right pane, too. So tell us what it says there.
Normally each device is represented by one horizontal block with a label block at its left end. There you will see a device name like "Disk_1", a type, a size, and a status. To the right in the larger rectangle are sub-blocks representing one or more Partitions on the HDD. Each of these will have a name assigned when the Partition was created, like "Harry's Disk", a letter name like D: assigned by Windows, a Partition size, a File Type like "NTFS" or "FAT32", and a status. If your unit has one Partition, and all of the label data is correct EXCEPT that is does not have a letter name assigned, you can fix that. You RIGHT-click on the Partition and choose to assign it a letter. When done, back out and reboot to update the Registry.
However, if the Partition shows you a File Type of "RAW", there is some corrupted data in the Partition Table or Directory system files, and Windows cannot deal with that. The good news, though, is that in most such cases the data are all there, and the error is only in the system files. In these cases you can use data recovery software like Easeus or GetDatBack to find all the HDD's files and copy them to another HDD. Note that you do need space on a second HDD to accommodate all those copies, but I think you intended that anyway. Once you are SURE you have all the files recovered, you can just use Disk Management's tools to Delete the Partition(s) on that old unit and re-Partion and Format it to re-use.