I like to use the cheatcode tohd to put the knoppix dvd onto the hard drive. However, I didn't use mkimage when I ran it initially. (I don't know if that would make a difference)
For some reason, using the cheatcode "mkimage" later was ignored.
My boot command line was something like this:
knoppix64 tohd=/dev/sda1
and it dutifully created the KNOPPIX directory on /dev/sda1 but I didn't get a persistent storage file.
One thing that's interesting is that when knoppix boots it automagically scans the drives and finds that KNOPPIX directory even if I don't specify the cheatcode "fromhd=/dev/sda1".
So if you want to create that knoppix-data.img by hand just launch a terminal after booting into your KNOPPIX and
dd if=/dev/zero of=/mnt-system/KNOPPIX/knoppix-data.img bs=1024K count=4000
This will make a 4000MB persistent file (you can make larger if you like) The hard drive that I'm using is formatted FAT32 so my limit is 4GB. Using other formats will have no such restriction.
(And as always, exercise care with dd since you can easily clobber your drives if you enter the wrong parameters. They don't call it disk destroyer for nothing. I've done it once or twice by entering parameters without thinking.)
Then you need to format it
mkfs.ext2 /mnt-system/KNOPPIX/knoppix-data.img
(You may need to use a -F parameter to "force" it to use a regular file, since it normally expects block devices)
(For some reason I thought it had to be reiserfs, but ext2 seems to work. I kept formatting it as reiser and it wouldn't use it)
Also, if you don't like the messages about KNOPPIX-DATA when you boot, there's a file called "knoppix-data.inf" that you can rename. It will mount a persistent PARTITION, and since we're using a persistent FILE, we don't really need it. The error messages don't hurt anything, but if you rename that file, you won't see them anymore. You can always rename it back to the original name.
mv /mnt-system/KNOPPIX/knoppix-data.inf /mnt-system/KNOPPIX/was-knoppix-data.inf
Gigabyte AMD B550 UD AC Gaming Motherboard - AMD B550 Chipset - AM4 Socket - AMD
$79.99
Gigabyte Z370P D3 ATX Z370 LGA1151 Motherboard (Support Intel 6/7th 8th 9th)
$59.99
ASUS H110M-R Motherboard Intel 6th/7th Gen LGA1151 DDR4 Micro-ATX i/o shield
$42.00
Motherboard AM4 A520m 4 slots Ryzen Gigabyte D3SH - Best price
$50.00
[REFUR] MSI B450M-A PRO MAX AM4 AMD B450 SATAIII USB3.2 Micro-ATX Motherboard
$55.99
ASUS Prime B250M-C LGA1151 DDR4 Desktop Motherboard USB 3.0 w/ I/O Shield
$39.99
MSI PRO B650-P WiFi AMD AM5 DDR5 WiFi 6E ProSeries Motherboard
$96.01
Intel DH61CR LGA1155 MicroATX Motherboard Core i3 i5 i7 Pentium NO CPU Fan HS
$14.99
MSI PRO B650-P WiFi AMD AM5 DDR5 WiFi 6E ProSeries Motherboard
$80.71
Asrock Z390 Phantom Gaming 4S/AC Wifi 8th/9th Gen Intel 1151 Motherboard Bulk
$44.74