Changing clock on RPi3B+ causes SD card to stop working

[John Klos <john%klos.com@localhost>]

Hi, ARM fans!

The SD cards in all of my various Pis work fine, except one. In a 
Raspberry Pi 3B+ (the 1400 MHz one), a 64 gig Samsung card works fine:

sdmmc0: direct I/O error 5, r=6 p=0xffffc000aa83ee3c write
sdmmc0: SD card status: 4-bit, C10, U3
ld0 at sdmmc0: <0x1b:0x534d:GC2QT:0x30:0xd5e35358:0x129>
ld0: 61056 MB, 7783 cyl, 255 head, 63 sec, 512 bytes/sect x 125042688 sectors

But when I use this cheaper card, switching from 600 MHz to 1400 MHz 
immediately makes accessing the card fail:

sdmmc0: direct I/O error 5, r=6 p=0xffffc000aa7c6e3c write
sdmmc0: SD card status: 4-bit, C6
ld0 at sdmmc0: <0x1d:0x4144:SD   :0x10:0x25700910:0x091>
ld0: 7646 MB, 3883 cyl, 64 head, 63 sec, 512 bytes/sect x 15659008 sectors

[    17.592114] ld0c: error reading fsbn 0 (ld0 bn 0; cn 0 tn 0 sn 0), retrying
[    18.092132] ld0c: error reading fsbn 0 (ld0 bn 0; cn 0 tn 0 sn 0), retrying
[    18.592151] ld0c: error reading fsbn 0 (ld0 bn 0; cn 0 tn 0 sn 0), retrying
[    19.092172] ld0c: error reading fsbn 0 of 0-2 (ld0 bn 0; cn 0 tn 0 sn 0), retrying
[    19.592190] ld0c: error reading fsbn 0 of 0-2 (ld0 bn 0; cn 0 tn 0 sn 0), retrying
[    20.092208] ld0c: error reading fsbn 0 of 0-2 (ld0 bn 0; cn 0 tn 0 sn 0), retrying
[    20.592226] ld0c: error reading fsbn 0 of 0-2 (ld0 bn 0; cn 0 tn 0 sn 0), retrying
[    21.092244] ld0c: error reading fsbn 0 of 0-2 (ld0 bn 0; cn 0 tn 0 sn 0), retrying
[    21.592262] ld0c: error reading fsbn 0 of 0-2 (ld0 bn 0; cn 0 tn 0 sn 0)
[    21.602263] ld0: disk label read failed

This system has root on USB but machdep.cpu.frequency.target=1400 was set 
and the EFI partition on the card was in fstab.

I can slow the clock if I want to access the card, but I wonder if 
something needs to be fixed, and therefore this information might be 
helpful to someone.

Thanks!
John Klos