Plugged in ext4 to Windows, how to recover the data in Ubuntu?

I moved all my data to a new drive and it seems to be lost. Please help me to recover the data.

Here is what I did to make the problem occur:
I moved, not copied the data. (First mistake) The HDD was making noises when I moved the data. Since the drive is new and the SMART values all looked good, I didn't worry too much about it, but maybe I should have?
To use the data I plugged in the drive into a Windows machine. (Second mistake) I tried to use DiskInternals LinuxReader to be able to read the ext4 format. Some files were readable other folders threw an error or appeared empty. I plugged in the drive into the original Ubuntu machine, only to see the same results.

What I tried to fix it:

• The first recommendation was to make a dd backup of the entire disk. I did that.
  Although when checking afterwards with df -h the original disk showed 981G used and the copy only showed 80K in use. So maybe something there went wrong?

• This morning I tried to continue after a reboot and suddenly I couldn't mount either disk:

  special device /dev/sdb1/ does not exist (a path prefix is not a directory).
  I tried what was suggest here and run sudo blockdev --rereadpt on both disks, but it ran without any feedback and the mount was still not possible. Since I planned on using e2fsck, I didn't worry about that too much, since I had to use an unmounted disk anyway.

• I ran gdisk with r to get to recovery options and then with b and c to read and recover the partition table from the secondary table. Since v gave no errors, I confirmed the write with w.

• Since that didn't work, I finally tried e2fsck. I confirmed everything it asked me to. Mostly invalid checksums, empty or "garbage" inodes and wrong counts for groups. Everything I read said this process might take hours, but it only ran ten minutes.

After that the mount finally worked, but df -h shows only 6.9G (There should be 981G) in use. The drive appears to be empty apart from around 50 entries in lost+found, so not nearly enough to cover all the files lost.

I don't know how to proceed and can't seem to find more answers on my own.
Why weren't the drives mountable anymore? - Answered by @mchid, thanks!
What can I try next, to have a chance at recovery of the data?

And if someone has the patience to educate me:
Did plugging the ext4 drive into a windows machine corrupt it? If so, why? From what I understood with the correct drivers (i.e. LinuxReader) it should be readable.

Any reply is greatly appreciated. Thanks for any help in advance!

Edit to add:

lsblk output:

NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda      8:0    0    8G  0 disk
├─sda1   8:1    0    1M  0 part
└─sda2   8:2    0    8G  0 part /
sdb      8:16   0  3.7T  0 disk
└─sdb1   8:17   0  3.7T  0 part
sdc      8:32   0  3.7T  0 disk
└─sdc1   8:33   0  3.7T  0 part
sdd      8:48   0 1000G  0 disk
└─sdd1   8:49   0 1000G  0 part /data

parted output:

(parted) print
Model: QEMU QEMU HARDDISK (scsi)
Disk /dev/sdb: 4001GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name  Flags
 1      1049kB  4001GB  4001GB  ext2

The mount problem is that /dev/sdb1/ refers to a directory, not a device.

The correct file path to the device is /dev/sdb1

The error: special device /dev/sdb1/ does not exist (a path prefix is not a directory) says that /dev/sdb1/ is incorrect because you have a trailing forward slash at the end which indicates a path to a directory.

When you run the mount command, use /dev/sdb1 instead of /dev/sdb1/.