Logical drive keeps disappearing

One of partitions on my hard drive keeps disappearing (or rather converting itself to something unusable). May or may not have something to do with my connecting an old boot hard drive to the new system to transfer data. Tried numerous things (learned DISKPART pretty well) but to no avail so far. I'll try to provide the most relevant information below but happy to supply more if necessary.

Goal: Need 3 logical drives on my hard drive (my usual setup on all home machines, never gave me troubles in the past). The 1 TB hard drive on my system came partitioned into a 990 GB system partition and a 10 GB of what I assume to be a recovery partition.

Steps taken before problem noticed: in Disk Management console shrunk system partition to some 330 GB, created a logical drive 330 GB in size (letter I by default as other letters apparently taken by DVD and card reader, that gave me another primary partition), created another logical drive from the rest of disk space (J in an extended partition). Both new disks usable and I have actually put some stuff on I drive straight away. Connected the old hard drive to transfer data. Possibly made a critical mistake here as the system tried to boot up as XP from the old drive, resulting in BSOD and reboot. Reconnected the old drive using an enclosure and found that it took the I letter. In Disk Management the former I drive was reported as healthy primary partition, no disk letter and the only right-click option to delete volume. J drive was in place and behaving happily.

Problem description: any attempts to create a logical drive in the area next after system partition result in the same scenario: the partition gets created and can be used quite happily, but after reboot it becomes a primary partition with no drive letter and apparently no access to data. I have tried numerous ways to get rid of the "bad" partition and re-claim that disk space, simply deleting and re-creating being the most obvious one. Just to mention some more: extend the system partition and shrink it again; fully format the partition under DISKPART; remove the extended partition, create a new extended partition that claims the entire space in DISKPART and then create 2 logical drives in it. Same story over and over, absoultely doesn't matter if the old disk is present on the system. In the last described scenario both new drives were gone and the extended partition became a primary one, not really what I expected to happen. Speaking in DISKPART terms, whatever happens to be Partition 2 on that physical disk becomes an empty primary partition on reboot full stop.

Any suggestions???

I assume I should try creating a placeholder logical disk of a minimum possible size, let it become the "dead" partition and then create my 2 logical drives in an extended partition the system will generate. Will probably do it this way as I need the system up and running. My real concern is that I don't understand what is going on and I will not feel the data in my logical drives, even if they don't disappear on every reboot, are safe in any way. Therefore any real fix to the problem, ideally with an explanation of what exactly went wrong so I can avoid it in the future, will be very much appreciated.

Thanks in advance,

Andrei