SD card problem - can't find a way to format it and test it

Question

I have this SanDisk Ultra 32GB HC I - class 10 SD card that I have been using on my android phone as means to install and run more stuff on my phone.

I have been using it for around 4 months and yesterday, when I woke up and tried using my phone, it started crashing (it was working fine the night before).

Every app installed in it took a long time to load, and would display error messages related to the unresponsiveness.

I figured the SD card got some problem that degraded it's performance, so I formatted it on android to be used as a "normal" storage card. I am thinking it didn't format properly.

I am trying for 2 days now, to format it in FAT and run a couple of diagnostics to give to SanDisk customer support, so maybe they can send me a new one.

I can't seem to be able to format it.

Using Ubuntu disk utility, it show the card partitioning information as "Master boot record". On the volume window, it shows as FAT.

Eventually, after running sudo dd if=/dev/zero of=/dev/mmcblk0 and mkusb:

1024+0 records in
1024+0 records out
1048576 bytes (1,0 MB, 1,0 MiB) copied, 0,45646 s, 2,3 MB/s
Error: Input/output error during write on /dev/mmcblk0
Warning: Error fsyncing/closing /dev/mmcblk0: Input/output error
Error: You requested a partition from 1049kB to 31,9GB (sectors 2048..62333951).
The closest location we can manage is 1048kB to 1048kB (sectors 2047..2047).
Error: Input/output error during write on /dev/mmcblk0
Warning: Error fsyncing/closing /dev/mmcblk0: Input/output error
mkfs.fat 3.0.28 (2015-05-16)
/dev/mmcblk0p1 has 64 heads and 32 sectors per track,
hidden sectors 0x0000;
logical sector size is 512,
using 0xf8 media descriptor, with 7997296 sectors;
drive number 0x80;
filesystem has 2 32-bit FATs and 8 sectors per cluster.
FAT size is 7795 sectors, and provides 997709 clusters.
There are 32 reserved sectors.
Volume ID is 39d99a2f, volume label            .
 Done :-) 
 created MSDOS partition table and FAT file system 
Unplug and re-plug the flash card to see the correct info ... and press Enter 
Wait 5 seconds and a little more ...
Model: SD SB32G (sd/mmc)
Disk /dev/mmcblk0: 31,9GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags: 

Number  Start   End     Size    Type     File system  Flags
 1      1049kB  31,9GB  31,9GB  primary  fat32        boot, lba


MODEL NAME        FSTYPE LABEL
      mmcblk0            
      └─mmcblk0p1 vfat   

This time, though Disk utility still shows the same, I was able to run some tests:

~$ sudo hdparm -t /dev/mmcblk0

/dev/mmcblk0:
 Timing buffered disk reads:  84 MB in  3.00 seconds =  27.99 MB/sec
~$ sudo hdparm -t /dev/mmcblk0

/dev/mmcblk0:
 Timing buffered disk reads:  86 MB in  3.06 seconds =  28.14 MB/sec
~$ sudo hdparm -t /dev/mmcblk0

/dev/mmcblk0:
 Timing buffered disk reads:  86 MB in  3.05 seconds =  28.22 MB/sec



~$ dd count=1k bs=1M if=/dev/zero of=/media/danilo/694C-0649/test.img
1024+0 records in
1024+0 records out
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 13,6004 s, 78,9 MB/s
~$ dd count=1k bs=1M if=/dev/zero of=/media/danilo/694C-0649/test.img
1024+0 records in
1024+0 records out
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 8,1578 s, 132 MB/s
~$ 

Which I guess is bad, as they mention it's speed as upwards to 80MB/s.

After all this I tried putting it back in my phone and formatting it as internal storage, but android says: command '21 volume partition disk:179,64 private' failed with '400 21 Command failed' And now my card has a 4.1GB partition somehow.

In summary, my phone started acting weird, I took the SD Card out, wasn't able to format is properly, might have a performance problem, can't figure out a way to test it for problems, Ubuntu's disk utility sees it as "partitioning MBR" BUT I can transfer files to the card.

How can I fix this issue, run some diagnostics for bad sectors and such, and properly measure it's performance?

EDIT: mkusb:

When I try executing the benchmark from Disk utility, I get: Error writing 10485760 bytes at offset 0: Input/output error (g-io-error-quark, 0)

Also, I don't think those labels(boot and lda) should be there but I can't seem to be able to remove them.