With MBR the very first sector is the boot code & the partition table. If you wrote some data into the MBR all partition tools may have issues.
If you have backed up all data, you can zero out the MBR.
I rarely suggest dd as its other nickname is Data Destroyer. Any typo and you may erase the wrong thing and there is no way to recover. Boot code is in first 440, entire boot sector including partition table is 512.
Confirm drive is sdb, as wrong drive erases partition table. if= is from, of= is to, do not ever mix them up.
Zero out MBR only of sda Use 440 if windows as serial number is between 440 & 446. Use 512 to include entire sector:
Some of info above is for reference in case someone else reads thread later.
dd if=/dev/zero of=/dev/sdb bs=512 count=1
Powerful command, but often misused and then nicknamed "dd" Data Destroyer